Question: How many 4-letter words with at least one vowel can be constructed from the letters A, B, C, D, and E?  (Note that A and E are vowels, any word is valid, not just English language words, and letters may be used more than once.)
Explanation: First we count the number of all 4-letter words with no restrictions on the word. Then we count the number of 4-letter words with no vowels. We then subtract to get the answer.

Each letter of a word must be one of A, B, C, D, or E, so the number of 4-letter words with no restrictions on the word is $5\times 5\times 5\times 5=625$.  Each letter of a word with no vowel must be one of B, C, or D. So the number of all 4-letter words with no vowels in the word is $3\times 3\times 3\times 3=81$.  Therefore, the number of 4-letter words with at least one vowel is $625-81=\boxed{544}$.